1. Field of the Invention
This invention relates generally to focal plane arrays, and more particularly to reference pixels for focal plane arrays
2. Description of the Related Art
Microelectromechanical systems (MEMS) are integrated micro devices or systems combining electrical and mechanical components. Some MEMS devices may be fabricated using standard integrated circuit batch processing techniques and have a variety of applications including sensing controlling and actuating on a micro scale. MEMS devices may function individually or in arrays to generate effects on a macro scale.
Certain MEMS devices require a vacuum environment in order to obtain maximum performance. The vacuum package also provides protection in an optimal operating environment for the MEMS device. Examples of these MEMS devices are infrared MEMS such as bolometers. In addition to the necessity of a vacuum or otherwise controlled environment for an infrared bolometer, infrared MEMS devices require an infrared-transparent cover, or lid structure. These lids are often coated with an anti-reflective coating to reduce the reflective properties and increase the infrared transmission properties of the lid.
Over the years, various types of infrared detectors have been developed. Many include a substrate having thereon a focal plane array, the focal plane array including a plurality of detector elements that each correspond to a respective pixel. The substrate contains an integrated circuit which is electrically coupled to the detector elements, and which is commonly known as a read out integrated circuit (ROIC) and which is used to integrate the signal from each detector element and multiplex the signals off the chip with appropriate signal conditioning and processing.
Each detector element includes a membrane which is suspended at a location spaced above the top surface of the substrate, in order to facilitate thermal isolation. The membrane includes a thermally sensitive material, such as amorphous silicon (a-Si) or vanadium oxide (VOx). The membrane also includes two electrodes, which are each coupled to the thermally sensitive material, and which are also coupled to the ROIC in the substrate. As the temperature of the thermally sensitive material varies, the resistance of the thermally sensitive material also varies, and the ROIC in the substrate can determine the amount of thermal energy which has been received at a detector element by sensing the corresponding resistance change of that detector element.
Uncooled infrared focal plane arrays operating at ambient temperature and without the use of active temperature stabilization require infrared optically blind reference pixels that do not absorb incident infrared radiation. These infrared optically blind reference pixels are used to determine ambient temperature of the focal plane which is required in the calibration of the focal plane array over the operating temperature of the focal plane array. This involves implementation of a gain and offset correction algorithm at any given temperature (sensed by the reference pixels) to the active detector elements in order to correct the image for ambient temperature drift effects, e.g., in an imaging focal plane array.
In the past, reference pixels have been made infrared optically blind by using short thermal isolation legs for the suspended infrared absorbing element in combination with placement of an infrared reflecting aluminum metallization directly on the reference pixel. However, reference pixels fabricated in this way have less Joule heating (e.g., in a voltage-biased suspended microbolometer structure) due to the shorter legs and to a lesser extent the added thermal mass of the aluminum reflector. As such, the output of the reference pixel as read out using a ROIC tends to diverge from the output of the infrared responsive suspended pixel detector elements, thereby limiting dynamic range both in terms of scene temperature and ambient temperature operating ranges.